North Korea's deputy UN ambassador has warned that the situation on the Korean peninsula "has reached the touch-and-go point and a nuclear war may break out at any moment".

Kim In-ryong told the UN General Assembly's disarmament committee that North Korea is the only country in the world that has been subjected to "such an extreme and direct nuclear threat" from the United States since the 1970s - and said the country had the right to possess nuclear weapons in self-defence.

He pointed to large-scale military exercises every year using "nuclear assets" and said what was more dangerous was what he called a US plan to stage a "secret operation aimed at the removal of our supreme leadership".

This year, Mr Kim said, North Korea completed its "state nuclear force and thus became the full-fledged nuclear power which possesses the delivery means of various ranges, including the atomic bomb, H-bomb and intercontinental ballistic rockets".

"The entire US mainland is within our firing range and if the US dares to invade our sacred territory even an inch it will not escape our severe punishment in any part of the globe," he warned. His speech follows escalating threats between North Korea and the United States, and increasingly tough UN sanctions.

Russian President Vladimir Putin said his country was curtailing economic, scientific and other ties with North Korea in line with UN sanctions, and the European Union announced new sanctions on Pyongyang for developing nuclear weapons and ballistic missiles.

US Secretary of State Rex Tillerson said this week that diplomatic efforts aimed at resolving the North Korean crisis "will continue until the first bomb drops".

His commitment to diplomacy came despite President Donald Trump's tweets several weeks ago that his chief envoy was "wasting his time" trying to negotiate with North Korean leader Kim Jong-un, whom he derisively referred to as "little Rocket Man".

North Korea's deputy UN ambassador called his country's nuclear and missile arsenal "a precious strategic asset that cannot be reversed or bartered for anything".

"Unless the hostile policy and the nuclear threat of the US is thoroughly eradicated, we will never put our nuclear weapons and ballistic rockets on the negotiating table under any circumstances," Mr Kim said.

He told the disarmament committee that North Korea had hoped for a nuclear-free world.

Instead, Mr Kim said, all nuclear states are accelerating the modernisation of their weapons and "reviving a nuclear arms race reminiscent of [the] Cold War era".

He noted that the nuclear weapon states, including the United States, boycotted negotiations for the Treaty on the Prohibition of Nuclear Weapons that was approved in July by 122 countries at the UN.

Meanwhile, a unit of US special forces tasked with carrying out "decapitation" operations may be aboard a nuclear-powered submarine docked in the South Korean port of Busan, the nation's newswire reported, citing a defence source.

The USS Michigan arrived in Busan ahead of a 10-day joint US-South Korean drill led by the USS Ronald Reagan aircraft carrier.

The US navy maintains that the Michigan, known for carrying special-ops teams, is docked in a "routine port visit". The US military also denies training for decapitation missions or regime change, and does not typically comment on Navy Seal deployments.

North Korea's 33-year-old dictator Kim Jong-un is "extremely nervous" of clandestine assassination plots, according to South Korea's intelligence agency.